The present invention relates to fan arrangements and, more particularly, to fan arrangements having increased overall efficiency and means to control the fan performance characteristics over a wide range by altering the inlet air flow to the fan wheel.
Air handling systems, such as those used in manufacturing facilities and buildings, usually employ large fans or blowers as part of the system. The essential elements of a typical fan include a centrifugal-type impeller or fan wheel mounted for rotation in a fan casing and a single speed motor for driving the fan wheel, generally through a `V` belt and pulley arrangement. While the present invention is adapted for use with most, if not all, fan designs, it is particularly suited for use with those fan designs which have their rotating components mounted on a hinged door. These type of fan designs, known as "swing-out designs," include fans having a scroll-like fan casing in which the fan wheel axis of rotation is aligned along the horizontal and fans having a tubular fan casing in which the fan wheel axis of rotation is aligned along the vertical.
In the swing-out scroll fan design, a plate-like door is hinged along one of its edges to the scroll-like fan casing and carries the fan wheel on one side of the door, a horizontally aligned mounting shaft which extends through the door, and the motor which drives the shaft through a `V` belt and pulley arrangement. In the closed position, the door locates the fan wheel in the fan casing with the inlet side of the fan wheel in an axially overlapping relationship with an air inlet bell or funnel. Substantially all the inlet air is directed to the inlet side of the wheel with the axial overlap between the inlet funnel and the fan wheel provided to minimize the amount of air recirculated from the outlet side of the wheel to the inlet side. The door, when opened, permits convenient access to the rotating components for inspection, maintenance, and/or cleaning.
The swing-out tubular fan design includes a vertically-aligned cylindrical or tubular fan casing with a portion of the sidewall so fabricated that it functions as a semi-cylindrical door with one edge of the door hinged to the remaining portion of the fan casing. The fan wheel is mounted on a vertically-aligned shaft on the interior side of the door and the drive motor is mounted on the other side of the door with a `V` belt and pulley arrangement extending through the door. When the door is in its closed position, the fan wheel is located within the tubular fan casing with the inlet side of the fan wheel aligned with the air inlet bell or funnel. In one exemplary tubular fan design, the door cooperates with a roller and inclined and/or segmented track arrangement but the inlet side of the fan wheel does not axially overlap the inlet funnel when the door is in the closed position to minimize the amount of air recirculated from the outlet side of the fan wheel to the inlet side. The door, when opened, permits convenient access to the rotating components for inspection, maintenance, and/or cleaning.
The performance characteristics of the swing-out fan designs are determined by the size of the fan wheel, the clearance dimensions between the fan wheel and its casing, the amount of power delivered to the fan wheel by the motor, and the amount of backflow or recirculated air from the outlet side of the fan wheel to the inlet side. The recirculation is controlled, in part, by the diameter clearances and axial overlap between the fan wheel and the inlet funnel. In one centrifugal fan wheel design, the fan wheel inlet includes a short cylindrical extension which is formed at a diameter either larger than or smaller than the diameter of the inlet funnel so that the inlet funnel telescopes with respect, that is, axially overlaps, the cylindrical extension to control the recirculation or backflow. The axial overlap and diameter clearances between the fan wheel inlet side and the inlet funnel are limited, in both the swing-out scroll fan and the swing-out tubular fan design, by the hinged nature of the door upon which the rotating components are mounted.
In the swing-out scroll design, points on the inlet side of the fan wheel describe curved paths when the door is opened with the radius of each path being measured from the hinge axis. The respective diameters of the axially overlapping inlet funnel and the inlet side of the fan wheel must be selected such that the fan wheel clears and does not interfere with the inlet funnel when the door is opened. This clearance requirement physically limits the amount of axial overlap on swing-out scroll fan designs. Consequently, fans of this type have a recirculation flow which cannot be conveniently minimized.
In the tubular fan design, the inlet side of the fan wheel moves in a plane parallel to the plane of the inlet funnel when the door is open. In order to provide the axial overlap needed to minimize recirculation or back flow, various mechanical arrangements are provided to lower or telescope the inlet side of the fan wheel with respect to the inlet funnel. These arrangements include, for example, a roller and an inclined and/or segmented track arrangement or screw jack arrangements which lower the fan wheel inlet side relative to the inlet funnel to provide the axial overlap when the door is closed.
The performance characteristics of these types of fans, in addition to being affected by the axial overlap, are not generally susceptible to convenient control or variation; consequently, fans of this type are generally designed to operate at an optimum point rather than over a wide performance range. Occasionally, a need arises for performance control over a much wider range than normally available including stable performance control at reduced air flow rates. Fan performance can be changed, for example, by varying the fan wheel speed. This can be done by providing a variable speed drive motor or, in those fan designs utilizing `V` belt and pulley transmissions, by changing the respective diameters of the various pulleys. Variable speed drive motors are both complex and expensive, and the removal and substitution of pulleys is a time consuming operation.
When the performance characteristics of a fan are varied, it is possible that the air flow at certain points in the fan, such as the fan wheel impeller blades, can become turbulent or unstable thereby causing undesirable pressure pulsations, the magnitude of which is a complex function of the fan structure and air flow path. Operation of a fan in these unstable regimes, of course, reduces the overall fan efficiency and subjects the fan structure to unnecessary vibration.